Double Selectivity of Cycloaddition Opens a Way to 1,2-Oxazoline-Pyrrole Ensembles via Nitrile Oxide Cycloaddition to -Allenylpyrrole-2-Carbaldehydes.

Cycloaddition of 1,3-dipoles to allenes usually proceeds in a nonselective way. Herein, we report on an efficient metal-free cycloaddition of nitrile oxides to readily available -allenylpyrrole-2-carbaldehydes with formation of 1,2-oxazoline-pyrrole ensembles. The reaction features a high regioselectivity and occurs only at the -bond of the allene with a very specific direction of the 1,3-dipole attack. A possible reason for this can be the effect of the formyl group, which remains intact, thus opening up possibilities for further functionalization of the target compounds. The regioselectivity of the cycloaddition has been rationalized by quantum-chemical calculations.